the x # of inches per cube is a myth. port sizing is more relative to sub displacement capability...throw(xmax) times surface area(sd). whe i was building a box for 2 15" ethos, Quentin suggested 104 inches of port for a 10 cube box.
I use this one - http://www.carstereo.com/help/Articles.cfm?id=31
Quentin suggested I use 22mm for xmax(instead of the 30 mm max). But I was pursueing a flatter response curve...Tuning to 22hz. My avalanche build log in my signature covers alot of it.

Hell yeah! I have a 92 RCSB with a 4/6 in my shop. Bought it in 95. Had 3 JL 12w6's on a MTX Thunder 2300. would beat pretty good for what it was. Porting the bed was always my hope- but couldn't bring myself to cut it up. Definately Tuned in.
Whats the front stage plan? Mine had crappy 4x6's in the dash and rear pillars. had to cut holes for a 6.5 component set in the door- totally amateur ish back then.LOL